Polyimides in general are well known in the art to be useful for high temperature applications, since they have a glass transition temperature of about 300 degrees Celsius an above. Such polymers may be prepared in any number of ways, perhaps the most common method being a two-step process including reacting a dianhydride such as pyromellitic dianhydride (PMDA) with a diamine to form a soluble polyamic acid which is then cyclized, thermally or by chemical means to form a polyimide.
Such procedures have been employed in connection with fluorinated polyimides as shown, for example, in Rogers U.S. Pat. No. 3,356,648. Example 11 of the '648 patent discloses a method of preparing a polyimide from 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 2,2'-bis(4-aminophenyl) hexafluoropropane. Equimolar amounts of the diamine and dianhydride are stirred together in dioxane for about eighteen hours at room temperature to form a polyamic acid. To the polyamic acid is added acetic anhydride and a minor amount of beta-picoline. After stirring for about 15 minutes, without cooling, the mixture is poured onto a glass plate to form a gel film. The gel film is heated in an oven at 120.degree. C. for twelve hours, then heated two more hours at 250.degree. C. to form a polyimide film. The polyimide film thus produced is reportedly soluble in chloroform, benzene, dioxane and acetone.
Other fluorinated polyimides are disclosed in Rogers U.S. Pat. No. 3,959,350. In Example I of the '350 patent, a fluorinated polyimide is prepared by mixing equimolar amounts of the 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 4,4'-diaminodiphenyl ether in dimethylacetamide under a nitrogen atmosphere at room temperature. The intermediate polyamic acid is converted to the corresponding polyimide by adding beta-picoline and acetic anhydride.
Fluorinated polyimides prepared as above do not have the desired properties in terms of molecular weight, color and other parameters required for many applications and thus further work has been done in this field. For instance, in Landis et al. U.S. Pat. No. 4,645,824, there is disclosed and claimed a method of preparing high molecular weight fluorinated polyimides prepared by way of cresol solution. In the '824 patent, a method of preparing polyimides is described including the steps of step equimolar amounts of 4,4'-hexafluoroisopropylidene bis(phthalic dianhydride) and 2,2'-bis(3-aminophenyl) hexafluoropropane in cresol at room temperature. The mixture is heated to about 215.degree. Celsius for one hour to yield polyimide of molecular weight up to about 35,000, reportedly soluble in dimethylacetamide.
The above noted polymers, while exhibiting the temperature performance characteristic of polyimides, do not exhibit the properties desired in many end-uses.
It is accordingly an object of the invention to produce fluorinated polyimide compositions exhibiting superior performance in terms of mechanical properties, thermal stability, low dielectrics, optical properties, and other, more uniform characteristics.
Another object of the invention is to achieve shaped articles of suitable mechanical and optical properties for end uses in demanding environments.
Still further objects and advantages of the present invention will become readily apparent from the following description and claims.